The present subject matter constitutes an improvement of the subject matter disclosed in commonly owned U.S. Pat. No. 4,429,462, the disclosure of which is hereby incorporated into the present specification.
As stated in the ""462 patent, coilable metal rules are widely used and frequently employ a concavo-convex cross-section or other cross-sectional configuration which will stiffen the blade so that the user may single-handedly bridge a distance with the blade tip extended in a straight line. This enables the user to bridge doorways, shaftways, excavations and the like with the unsupported blade. It is known that the degree of curvature or cross-section will influence and substantially determine the standout length (the length of the blade which can be extended unsupported without collapsing at the support point) or breakpoint (the point at which the unsupported rule will collapse) for a rule of any particular width, thickness and metal characteristics.
Such blades are generally used in power returnable coilable rules that include a coiled spring which has the effect of retracting the extended blade into the rule housing or housing wherein it coils about the reel in its retracted position. It will be appreciated that concavo-convex cross-section of the blade is flattened as it is coiled within the rule housing, whether by manual action or by the retracting effect of such a power return spring. The stiffer the blade, the more spring power required to effect such flattening of the blade upon retraction.
As is also well known, blades employing such a concavo-convex cross-section generally must be heat treated to achieve the desired flexural characteristics to withstand the repeated flattening operation that occurs in the coiling of the blade within the rule housing. It has been necessary to balance the advantages to be gained by increasing the depth of curvature of the blade for increased length of blade standout with the power requirements for the spring to effect its retraction and also with the life characteristics resulting from repeated deflection into the flattened condition when the blade is coiled.
Moreover, because the extended blade will assume the concavo-convex cross-section, the indicia which are imprinted thereon become more difficult to read when the arcuate cross-section becomes steeper. Lastly, when there is a very high cross curve or deep cross-section, there is a stress point at the point where the blade does buckle when fully extended, which, over repeated usage, may cause a stress fracture.
Because of the necessity for balancing these various factors, the range of average standout length for commercially available widths of concavo-convex blades is set forth in the following table:
The manner of achieving a greater standout in the ""462 patent is to provide an intermediate portion of the blade having an extent along the length of the blade which includes the normal breakpoint for the blade and at least the increase in the standout contemplated which has a sharper arcuate extent than the remainder of the blade. The configuration of the intermediate portion is limited by the following statement found in the ""462 patent specification:
Various cross-sections have been used for the concavo-convex blade construction. Generally, the cross-section is a segment of a circular arc or a segment of a circular arc with straight segments at its ends (e.g., rectilinear portions which are tangential to the center arcuate segment). In this manner the concavo-convex cross-section will not have any sharp bends which would produce points where the blade will be overstressed during the repeated flattening operation, and the amount of force required to flatten the curve is more readily available from economically utilizable power return springs.
The practical effect of this limitation is to render the additional standout which can be practically achieved to an amount which may not justify the added costs involved to change the configuration of the intermediate portion.
The present application is based upon the proposition that providing the intermediate portion with a configuration beyond the limits of the ""462 patent intermediate portion configuration can have a practical material effect in extending the incremental standout without having a material effect on the size of the wrapped rule blade coil or the strength of the spring required to wrap the rule blade coil.
Accordingly, it is an object of the present invention to provide a coilable rule assembly having an increased standout comprising a housing defining a chamber having an outlet, an elongated rule blade, and a controllable spring biased reel assembly mounted within the chamber having one end of the rule blade connected thereto. The controllable spring biased reel assembly is constructed and arranged to wind the rule blade into a coiled state within the chamber and to allow the rule blade to be extended outwardly of the chamber outlet into an uncoiled state. The rule blade is formed of a strip of resiliently deflectable metal having a uniform width and thickness such that when extended into the uncoiled state the rule blade assumes a generally concavo-convex cross-sectional configuration with the concave side having indicia thereon and when wound in the direction of its concave side into the coiled state the rule blade assumes a flattened coil configuration. The rule blade has an end blade portion connected to the controllable spring biased reel assembly, a free end blade portion and an intermediate blade portion between the end blade portions.
The free end portion is constructed and arranged to stand out by itself from the housing with the concave side facing generally upwardly without bending into a flattened condition in the direction of the convex side thereof along a breakpoint while allowing progressive movement into a flattened condition in the direction of the concave side of the rule blade during winding thereof into the coiled state. The intermediate blade portion has an extent which includes the normal breakpoint for a rule blade of the width, thickness, cross-sectional configuration, and material of the free end blade portion thereof and at least the increase in the standout contemplated. The intermediate blade portion has a modified cross-sectional configuration with respect to the free end blade portion which is constructed and arranged to provide a greater resistance to bending into a flattened condition in the direction of the convex side of the rule blade than the free end blade portion so as to increase the standout of the rule blade while allowing progressive movement into a flattened condition in the direction of the concave side of the rule blade during winding thereof into the coiled state. The intermediate blade portion is formed with rib structure extending longitudinally therein when in an uncoiled state which resists bending in the direction of the concave side of the rule blade. The intermediate blade portion including the rib structure when in its flattened coiled configuration has a cross-section which lies within an elongated rectangle having a thickness slightly greater than the uniform thickness of the rule blade by virtue of the rib structure therein.
Preferably, the rib structure is in the form of a pair of generally parallel elongated shallow ribs equally positioned on opposite sides of the center line of the rule blade, although the broadest aspects of the invention contemplates a single centrally located shallow rib or more than two shallow ribs. Preferably, the ribs open in the direction of the concave side of the rule blade although it is also contemplated that they open in the direction of the convex side of the rule blade.
Preferably, the ribs are formed by the juncture between a central arcuate cross-sectional segment and outer cross-sectional end segments which are preferably straight although curved end segments are contemplated.